[PATCH v16 07/18] arm64: ptrace: Protect rseq_syscall() from tracer PC modifications

Move the rseq_syscall() check earlier in the syscall exit path to ensure
it operates on the original instruction pointer (regs->pc) before any
potential modification by a tracer.

[Background]
When CONFIG_DEBUG_RSEQ is enabled, rseq_syscall() verifies that a system
call was not executed within an rseq critical section by examining
regs->pc. If a violation is detected, it triggers a SIGSEGV.

[Problem]
Currently, arm64 invokes rseq_syscall() after report_syscall_exit().
However, during report_syscall_exit(), a ptrace tracer can modify the
task's instruction pointer via PTRACE_SETREGSET (with NT_PRSTATUS). This
leads to an inconsistency where rseq may analyze a post-trace PC instead
of the actual PC at the time of syscall exit.

[Why this matters]
The rseq check is intended to validate the execution context of the
syscall itself. Analyzing a tracer-modified PC can lead to incorrect
detection or missed violations. Moving the check earlier ensures rseq
sees the authentic state of the task.

[Alignment]
This change aligns arm64 with:
- Generic entry, which calls rseq_syscall() first.
- arm32 implementation, which also performs the check before audit.

[Impact]
There is no functional change to signal delivery; SIGSEGV will still be
processed in arm64_exit_to_user_mode() at the end of the exit path.

Cc: Mark Rutland <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Catalin Marinas <[email protected]>
Reviewed-by: Ada Couprie Diaz <[email protected]>
Reviewed-by: Linus Walleij <[email protected]>
Reviewed-by: Yeoreum Yun <[email protected]>
Reviewed-by: Kevin Brodsky <[email protected]>
Signed-off-by: Jinjie Ruan <[email protected]>
---
 arch/arm64/kernel/ptrace.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index ae71cadbd4df..bc08c93c69c5 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -2455,6 +2455,8 @@ int syscall_trace_enter(struct pt_regs *regs, unsigned long flags)
 
 void syscall_trace_exit(struct pt_regs *regs, unsigned long flags)
 {
+	rseq_syscall(regs);
+
 	audit_syscall_exit(regs);
 
 	if (flags & _TIF_SYSCALL_TRACEPOINT)
@@ -2462,8 +2464,6 @@ void syscall_trace_exit(struct pt_regs *regs, unsigned long flags)
 
 	if (flags & (_TIF_SYSCALL_TRACE | _TIF_SINGLESTEP))
 		report_syscall_exit(regs);
-
-	rseq_syscall(regs);
 }
 
 /*
-- 
2.34.1